The present invention relates to a technology for controlling the flying height of a magnetic head in order to prevent the magnetic head from colliding against a protrusion on the surface of a magnetic disk in a magnetic disk drive.
In a magnetic disk drive, a head/slider assembly containing a magnetic head flies slightly above a rotating magnetic disk. The magnetic head is configured as a composite magnetic head, which generally includes an inductive thin-film write head and an MR read head, which employs an MR (magnetoresistive) element. These two heads are formed on a single slider. The clearance between the magnetic head and magnetic disk surface is referred to as a flying height. To provide an increased recording density, it is necessary to minimize the magnetic head flying height to increase the sensitivity for detecting a change in a magnetic field generated by magnetic layer particles or decrease the size of the magnetization pattern to be recorded.
The MR read head detects a resistance change caused by an external magnetic field by reading a voltage that is dependent on a change in the current flowing to the MR element. When an inductive read head is used, a high detection sensitivity can be obtained. In a magnetic disk manufacturing process, it is difficult to make the magnetic disk surface completely flat. Therefore, a certain protrusion remains on the magnetic disk surface. The smaller the flying height, the higher the frequency with which the MR read head comes into contact with the protrusion. Thermal energy arises when the MR read head comes into contact with the protrusion. The thermal energy changes the resistance in accordance with its temperature coefficient. The resulting read signal then fluctuates so that an accurate read operation cannot possibly be performed. This phenomenon is known as a thermal asperity (hereinafter referred to as a TA).
If the write head comes into contact with a protrusion on the magnetic disk surface during a write, the head/slider assembly unsteadily flies so that servo control may be disturbed. To prevent the write head from writing into a track next to a target track and avoid a write error, a write operation does not start until servo control is stabilized. Therefore, if the write head comes into contact with a protrusion, the settling time increases. Further, if the write head or read head frequently comes into contact with a protrusion, the head life may be shortened or broken pieces of the protrusion may be scattered over the magnetic disk surface to increase the number of protrusions.
To eliminate a flying height margin that is provided to cope, for instance, with machining irregularities and barometric pressure differences, the invention disclosed by Patent Document 1 (Japanese Patent Laid-open No. 2002-150735) uses a piezoelectric element mounted on the head/slider assembly and TA to perform a proper shift amount estimation process and determines the amount of piezoelectric element control (see 0041 and 0042). The proper shift estimation process is then performed at the innermost circular position and outermost circular position to store in memory the optimum application voltages for the piezoelectric element at both positions. In an actual read/write operation, the magnetic head/slider assembly is moved to a target track while reading the servo information at a great flying height with no voltage applied. The proper application voltages stored in memory are then subjected to linear interpolation. The resulting voltage is finally applied to the piezoelectric element (see 0045).
The invention disclosed by Patent Document 2 (Japanese Patent Laid-open No. 10-233070) uses a TA to detect a magnetic spacing between the magnetic disk and magnetic head and uses an actuator, in which a piezoelectric element is incorporated, to provide a constant magnetic spacing (see 0016). Further, the piezoelectric-element-based actuator is used to follow a protrusion on the magnetic disk.
The invention disclosed by Patent Document 3 (Japanese Patent Laid-open No. 10-69747) optimizes the flying height prevailing at the time of track following while preventing the head/slider assembly from colliding against the magnetic disk due to an increase in the flying height. A piezoelectric element for controlling the head/slider assembly flying height is furnished. The correction amount is calculated from a flying height profile curve that is predetermined in accordance with the results of a simulation that is conducted in relation to the head/slider assembly acceleration and speed. A signal is given to the piezoelectric element to keep the flying height constant (see 0015).